1. Field of the Invention
The present invention relates to a compact taking lens system, and more particularly to a low-cost, compact taking lens system suitable for use in a digital input apparatus such as a digital still camera or digital video camera.
2. Description of the Prior Art
In recent years, as personal computers become prevalent, digital still cameras, digital video cameras, and the like (hereafter referred to simply as digital cameras) that allow easy storage of image data in a digital format are becoming more and more popular for personal use. Such digital cameras are expected to become further more widespread as an apparatus for inputting image data.
On the other hand, digital cameras have come to employ smaller and smaller solid-state imaging devices, for example, CCDs (charge-coupled device), and therefore digital cameras themselves are quite naturally expected to be made more and more compact. Consequently, further miniaturization is eagerly sought also in taking lens systems, because they occupy the largest space within a digital input apparatus. Furthermore, the recent market competition for lower prices demands accordingly inexpensive taking lens systems. To cope with such requirements, Japanese Laid-open Patent Applications Nos. H9-133859, H9-222555, and H10-48515 propose compact taking lens systems for use in a digital camera that are composed of as few as four to five lens elements in total. Japanese Laid-open Patent Applications Nos. H9-43512, H9-166748, and H10-20188 also propose compact taking lens systems for use in a digital camera that are composed of as few as five to six lens elements in total. Considering that silver-halide-film-based lens shutter cameras have recently come to employ increasingly and often strikingly compact taking lens systems, it is also possible to apply such taking lens systems to digital cameras.
The taking lens systems proposed in the above-mentioned patent applications are compact, but, because all of their constituent lens elements are made of glass, it is impossible to achieve cost reduction. On the other hand, even if a taking lens system designed for use in a lens-shutter camera is employed intact in a digital camera, it is impossible to make efficient use of the light-condensing ability of microlenses disposed on the front surface of a solid-state imaging device. This is because, in a taking lens system designed for a lens-shutter camera, the exit pupil is placed so close to the image plane that off-axial light rays exiting from the taking lens system are obliquely incident on the image plane. This makes efficient use of the light-condensing ability of the microlenses impossible, and, as a result, the image suffers from uneven brightness between the central and peripheral portions thereof. This problem can be solved by placing the exit pupil farther away from the image plane, which, however, cannot be realized without making the taking lens system as a whole unduly large.